SCIENCE sciencemag.org 6 DECEMBER 2019 • VOL 366 ISSUE 6470 1211
PHYSICS
Imaging heavy
Dirac fermions
Most topologically nontrivial
materials discovered to date
have negligible electron
correlations. Strongly cor-
related topological materials
are predicted to have a rich
phenomenology; however,
identifying such materials
and proving their topological
character has been tricky.
This is particularly true
of samarium hexaboride
(SmB 6 ), a compound that is
theoretically expected to be a
topological Kondo insulator,
but the multitude of probes
used to study it have yielded
conflicting results. Pirie et
al. applied scanning tun-
neling spectroscopy to the
problem and found evidence
of Dirac states with large
effective masses, which is in
agreement with theory. They
were able to reproduce the
results for multiple samples
synthesized by two different
growers. —JS
Nat. Phys. 10.1038/
s41567-019-0700-8 (2019).WORKFORCE
Graduate students
under pressure
Although graduate student
mental health is a topical issue,
there is little empirical research
on the subject. Hish et al. used
a stress process model to
examine the roles of mastery
and social support as mediators
of stress. Using a validated self-
report measure of stress, data
relating to stress-burnout and
stress-depression relationships
were collected from biomedi-
cal graduate students. Linear
regression models showed that
academic stressors were most
predictive of burnout, whereas
depressive symptoms were best
predicted by family and mon-
etary stressors. Furthermore, the
relationship between stress and
burnout was partially mediated
by mastery and advisor support,
whereas the stress-depression
relationship was partially medi-
ated by mastery, suggesting that
future interventions might focus
on enhancing mastery and/or
improving advisor relationships.
—MMc
CBE Life Sci. Ed. 18 , ar51 (2019).IMMUNOLOGY
Delicate dancing
in the liver
Kupffer cells are a subset of
macrophages lining the sinu-
soids of the liver. There, they
engage in specialized functions,
including the consumption and
breakdown of erythrocytes.
However, the interactions that
imprint the tissue-specific phe-
notype of Kupffer cells is poorly
understood. Two groups provide
insights into how macrophage
progenitors are directed toward
this lineage. Bonnardel et al. and
Sakai et al. depleted Kupffer
cells and then monitored the
macrophages that repopulated
the liver for clues. Their findings
indicate that there is a complex
interplay of Kupffer cells with
stellate cells, hepatocytes, and
sinusoidal endothelium in the
perisinusoidal space. These
other cell types work to recruit
monocytes and then induce
and maintain the Kupffer cell
phenotype via Notch–bone mor-
phogenetic protein signaling and
transforming growth factor–b
family ligands. —STS
Immunity 51 , 638, 655 (2019).
MATERIALS SCIENCE
Better fatigue
resistance at low cost
Framework or microarchi-
tectured materials can be
designed to have a combina-
tion of strength and stiffness,
not through changes in the
underlying material but through
variations in lattice density and
architecture. By looking at the
properties of cancellous bone,
which is better than compact
bone for stress dampening,
Torres et al. explored ways to
enhance the fatigue properties
of microarchitectured materials.
They found that a key element is
the proportion of materials ori-
ented transverse to the applied
loads, because these materi-
als act as sacrificial elements
during cycling loading. Thus,
although there is a tendency to
design framework materials to
maximize the struts oriented in
the direction of expected load-
ing, this can lead to a drastic loss
of fatigue resistance and only
minimal decrease in the overall
density. —MSL
Proc. Natl. Acad. Sci. U.S.A. 10.1073/
pnas.1905814116 (2019).MOLECULAR BIOLOGYEpigenetic plasticity
T
he Florida carpenter ant, Camponotus
floridanus, is a social insect with two worker
castes that, despite sharing the same
genome, are phenotypically distinct: “Major”
workers defend the nest, and “minor” workers
forage. Because major workers can be experimen-
tally reprogrammed into minor workers, they are
a great system to study the underlying epigen-
etic mechanisms for phenotypic differentiation.
Glastad et al. compared gene expression in the
brains of reprogramming-capable younger and
reprogramming-incapable older major workers,
as well as in the brains of major and minor work-
ers during reprogramming. Chromatin repressor
CoREST is up-regulated and required during
reprogramming to repress major-biased genes,
including enzymes that degrade juvenile hormone,
which is key for regulating caste specificity. Thus,
a chromatin-based mechanism that links transient
epigenetic plasticity to long-lasting and complex
social behavior has been uncovered. —SYM
Mol. Cell 10.1016/j.molcel.2019.10.012 (2019).Published by AAAS